In a number of research papers, suggestions are that it depends on age, duration and volume of smoking, but after 5 years the risk of primary lung cancer tends to match that of a non-smoker. Although the hazards of smoking until age 40 years and then stopping are substantial, the hazards of continuing are ten times greater. Stopping smoking before age 40 years (and preferably well before age 40 years) avoids more than 90% of the excess mortality caused by continuing smoking; stopping before age 30 years avoids more than 97% of it.
- Pirie K, Peto R, Reeves GK, Green J, Beral V; Million Women Study Collaborators. The 21st century hazards of smoking and benefits of stopping: a prospective study of one million women in the UK Lancet. 2013 Jan 12;381(9861):133-41.
Although a useful study, imagine the benefits of this combined with an improved diet along with exercise and stress reduction techniques. Stopping smoking can also be achieved more easily when stress levels are better managed.